Key-click eliminator circuit



June 17, 1941. G. w. CATTELL KEY-CLICK ELIMINATOR CIRCUIT Filed Oct. 17, 1938 2 Sheets-Sheet 1 TIME IN THOUSANDTHS OF SECONDS INVENTOR, GILBERT w. CATTELLL BY 2 ATTORNEYS.

Patented June 17, 1941 UNITED STATES PATENT OFFICE I 2,245,569 KEY-CLICK ELIMINATOR omcorr Gilbert W. Cattell, Berkeley, Calif. Application October 17, 1938, Serial No. 235,439

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 14 Claims.

My invention relates to communication. by radio, and more particularly the reception of code in the telegraphic transmission of messages.

The invention described herein, if patented, may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

In the transmission of code messages by radio, a considerable amount of interference is encountered which has its origin in the transmitting apparatus, and such interference is designated as key-clicks due to the fact that it is generated by the keying mechanism while in operation in the transmission of the dots and dashes forming the messages to be transmitted. Keyclick interference is not only annoying to those operators with receivers adjusted to receive the code messages under consideration but also constitute a source of interference to operators of receiving apparatus on adjacent frequency channels.- At times such interference may become so bad as to render unintelligible the messages received.

It is, accordingly, an object of my invention to provide an improved circuit for the elimination of key-click interference in the reception of messages by radio.

Another object of my invention is to provide an improved circuit for the reception and reproduction of code messages.

Another object of my invention is to provide improved means for the reception and reproduction of desired radio signals to the exclusion of undesired interference.

Another object of my invention is to provide a circuit of simple design capable of eliminating interference of the general character of key clicks from received messages.

A further object of my invention is to provide an improved circuit for the elimination of interference from received messages, which circuit may be conveniently added to existing apparatus.

Additional objects of my invention will become apparent from the following description of the same taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a circuit diagram reduced to its simplest form and embodying the features of my invention.

Fig. 2 embodies a series of curves illustrating the principles underlying the operation of my invention.

Fig. 3 is a circuit diagram, partly schematic,

illustrating the application of my invention to a diversity receiver.

My invention is based upon my observation that key-clicks, although they may be of an intensity considerably greater than the'dots and dashes constituting the message, are invariably of extremely short duration as compared to the duration of a dot when transmitted. Of course, this ratio will vary with the speed of transmission but I have found, nevertheless, that even at high speed transmission of the order of words per minute, the time period, or duration of a dot, which may be approximately .00833 of a second, is still considerably longer in time than the period of the average key-click which may run somewhat less than .00-1225 second. Thus, even at this high rate of transmission, the dot will represent a period of time lasting more than six times as long as the duration of the average keyclick.

Broadly speaking, my invention comprises a circuit designed to differentiate between signal of a time duration less than that representing a period slightly greater than the average keyclick, and those signals of greater duration than the same period. In this respect the circuit will differentiate between desired and undesired signals, and can therefore be made to respond only to the desired signals which will include the dots and dashes but which will not include the key-clicks or other impulses of the same order of time duration as key clicks. From another viewpoint, the circuit may beconsidered as responding only to signals of a predetermined minimum time duration, the predetermined minimum time being that value which will exclude the undesired interfering impulses.

Referring to Fig. 1, I have illustrated the bare essentials necessary to successful operation of my invention, .and for an understanding of the same reference will be made to this figure of the drawings and subsequently the application of the invention to a diversity receiver will be described.

The tube of Fig. 1 may constitute any amplifier tube of a radio receiving set. It may be in the radio frequency portion of the receiver or in the audio frequency portion, and will usually be one of the vacuum type having an anode 3, cathode 5, and a control electrode 1. The input circuit 9 to the tube will be connected between the control electrode 1 and cathode 5, and will usually include an input winding l I and bias voltage source l3, theinput winding being coupled or otherwise associated with the preceding portion of the receiver so as to have impressed upon the input circuit, the signals being received,

such signals including the undesired interfering impulses.

The output circuit of the tube l is connected between the anode 3 and the cathode 5 and includes a source of anode potential it, a second electron discharge device H, and a relatively high resistance iii, all in series with each other. The second electron discharge device ll has an anode 2i, cathode 23, and control electrode 25, the cathode being connected to the anode side oi: the first tube i with the anode 21 of the second tube connected to the cathode side of the first tube. The tube l l is normally biased by a source of biasing potential El connected between the control electrode 25 and the cathode 23 thereof, and in series with this biasing potential is a resistor 29 shunted by a condenser 32, both of which may be variable if desired for convenience of adjustment An additional resistance 33 of greater magnitude than the condenser shunted resistance 2% is connected from. the cathode 5 of the first tube l to a point in the grid cathode circuit of the second tube ll between the condenser shuhted resistance '29 and the biasing potential 2?. The condenser shunted resistance 29 and the resistance 33 connected thereto constitutes a shunting circuit about the second electron discharge device l1 and its associated resistance E9 connected in series therewith. The successful operation of the circuit described not only depends upon the presence and circuit arrangement oi the elements just mentioned, but also upon the adjustment of such elements to introduce into the circuit the proper time factor which will enable the circuit to difiererltiate between the undesired and the desired signals. The manner in which this time factor is introduced into the circuit and its importance in the successful operation of the invention will now be described.

To begin with, the presence of the shunting circuit including the resistance 29, condenser E! and resistance 33, will, for the time being, be disregarded, thus leaving the first tube l with its output circuit including only the anode potential supply source It, the second electron discharge device l'i' and the resistance [9, in series therewith, and a negative bias from voltage source Eli sufficient of itself to block the tube ll. With the aforementioned output circuit thus stripped of the shunting circuit about the second electron discharge device l'i, the second electron discharge device il will present substantially infinite impedance to the flow of plate current through the first tube l, and consequently, regardless of the application of signals to the input circuit 8 of the first tube, no current will flow in the plate circuit of the first tube.

The presence of the shunting circuit, however, alters conditions and provides a path for the how of plate current or anode current around the second electron discharge device I7 and its associated resistance l9, and thus permits the first tube II to pass plate current in response to signals even though the second discharge device ll be biased to cut off. The purpose and operation of the shunting condenser 3! will for the moment be disregarded, and in its absence from the shunting circuit the first electron discharge device I, in responding to incoming signals, will send plate current through the shunting circuit resistances '29 and 33, and practically no current will pass through the second device I? due to its high internal impedance while blocked by the negative bias from the source 21. The resistance 29 in the grid circuit of the second electron discharge device H is of such a value that this plate current will instantaneously build up a voltage thereacross of a Value suflicient to unbias the second electron discharge device H and render it conductive, and thereafter the signal current in the plate circuit of the first electron discharge device will find a low resistance path through the second electron discharge device ii, and the resistance E9 in series therewith. Without the shunting condenser, the breaking down of the second electron discharge device is, as previously stated, practically instantaneous, as the unbiasing resistance offers no time delay in the unbiasing of this second electron discharge device.

The condenser 3| when connected to shunt this resistance will serve to introduce a time factor, and the value of such condenser is so determined, that the time delay introduced in the breaking down of the second electron discharge device IT will be of a time duration slightly greater than the duration of the interfering signals such as the key-clicks, etc., which it is desired to eliminate. It will be apparent that with the circuit adjusted to introduce this time factor, all signals impressed upon the input circuit 9 of the first electron discharge device 1 which are of a time duration less than the time delay factor introduced into the operation of the circuit, will not appear across the resistance l9 in series with the second electron discharge device H, for the simple reason that the duration of such undesired signal will not permit the building up of a potential across the unbiasing resistor 29 suificient to unbias the tube and render it conductive. In other words, the interfering impulse appearing in the signal input circuit of the first electron discharge device, will have disappeared before the potential building up across the condenser shunted resistor 29 will have reached a value sufiicient to unbias the second electron discharge device and permit the same to pass a signal.

Accordingly, across the resistor, in series with the second electron discharge device, will appear only signal potentials of a time duration greater than the key-clicks or other interfering impulses of comparable time duration, and consequently, the dots and dashes will appear as voltages across this resistance It to the exclusion of the interfering impulses. Should a signal translating device such as a loud speaker or recorder or the like he made responsive to the potentials developed across the resistance IS, then it will become apparent that such device will translate the desired signals to the exclusion of the undesired interference signals.

The second electron discharge device l! is preferably one of the gas filled type, because such a tube possesses the properties, when unbiased, of changing suddenly from a non-conductive to a conductive condition, and a sudden transformation of this character is highly desirable in the formation of properly shaped impulses for operation of the signal interpreting apparatus, it being noted that for best operation of such apparatus the dot and dash impulses transmitted thereto shouldpreferably be of square-top waveform. In a tube of the gas-filled type, after it has once become conductive, the grid electrode no longer exerts any influence on the current flowing through the tube, the magnitude of which current is determined by the anode potential of source l5 and the external resistance of the circuit. The current continues to flow until the plate potencircuit'of the first electron discharge device.

In the meantime, upon completion of the dot I or dash signal and the gas filled device ll becomes nch-conductive again in the manner described, the condenser 3! will discharge through resistance 2% and prepare itself for the succeeding signal. In the event of a key-click or other undesired signal of like time duration, the condenser 3l, which will have assumed a partial charge, insufficient to unbias the tube II, will likewise discharge upon the termination of such undesired signal, the only difference being that the beginning of. discharge will not have to await the extinguishment of tube H, for this tube will not have become active under such circumstances.

In Fig. 2, there appears a family of curves illustrating the theory underlying the adjustment and operation of the condenser-shunted unbiasing resistor 29. The horizontal axis is marked off in terms of thousandths of seconds, whereas the vertical axis is marked ofi in terms of a ratio of the voltage across the unbiasing condenser shunted resistor 28 and the anode voltage source i5. Each of the curves represents the building up of the unbiasing potential across a predetermined constant value of resistance for different values of shunting condenser. In determining which value of. condenser will be most satisfactoryfor use with a resistance of a predetermined value, the

minimum time duration period which will distinguish the desired from the undesired signals must be taken into consideration. If, for the particular resistance employed, a voltage representing six per cent of the anode plate potential will unbias the second electron discharge device, then that curve will be selected which reaches this unbiasing potential value at approximately the aforementioned minimum time period.

With reference to the curves illustrated, it will be noted that the curve for a condenser of .01

microfarad reaches the proper unbiasing potential in .0005 second and the other curves in the order shown, reach the same value of potential in .002 second, .0015 second, .002 second, and .0025 second, respectively. If the key-click or other interfering impulse which it is desired to eliminate, is of less time duration than .00125 second,

for example, then a curve will be selected which will lie between the .001 second curve and the curve representing a time factor of .0015 second; and the condenser which will provide such a time delay factor will accordingly have a value of between..02 microfarad and .03 microfarad. With an adjustment such as described, high speed keying of the order of 150 words per minute can be received to the exclusion of key-click or similar interference. For the reception of slower speed keying the determination of the condenser value will not be quite as critical as for operation at higher speeds, by reason of the fact that the dots and dashes will be of longer time duration.

Similar curves varying in slope characteristics may be obtained by varying the resistance 29 and maintaining a constant value of shunting capacity, or both may be varied, thus affording a wide range of curves from which to select proper values of the condenser 31 and the resistance 29. The time delay may also be varied by varying the bias potential 21 on the second electron discharge device [7, corresponding to adjusting the cut off voltage to above or below that indicated as .06 in Fig. 2.

The above series of curves have been plotted, utilizing the customary exponential formulas for the charging and discharging of a condenser, and in employing such formulas it was assumed that an audio-signal would apply and cut off a D.-C. potential throughout the duration of the signal. This is not strictly the case, as only a pulsating uni-directional potential is applied to the device H, the resistance 29 and its shunting condenser 3|, and the resistance 33, all in series.

A careful comparison of the charge and discharge curves of Fig. 2 reveals that for uniform 1/2 cycle square-top impulses, the condenser 31 tends to charge slightly more rapidly than it discharges, and that eventually it would build up to a value of the time required being greater than that calculated. Y

However, a condenser 35 is connected between thecathode of the second discharge device I! and ground, which condenser charges almost instantaneously through the plate impedance of the first discharge device I, which is quite low when an audio-signal is impressed on the input circuit 9 of the first discharge device I. The only discharge path for the condenser 35 is through the resistance 29 and the resistance 33 in series, thus causing condenser 35 to discharge more slowly than it charges. The effect of the rapid charge and slow discharge of the condenser 35, makes the potential applied to the resistance 29 andshunting condenser 3| and the resistance 33 more nearly approximate a constant or direct current flow.

The frequency of the charging impulses which, in an actual application of the invention to a receiving set, would probably be determined by a heterodyne beat note, has an important bearing on the approximation to a constant direct current supply potential to the eliminator circuit of Fig. 1. The higher the beat frequency, the more nearly will the signal potential applied to the eliminator circuit approach a uniform Da-C. voltage value. It has been noted in practice that the higher the frequency of the input signal, the

greater the discriminating power of the eliminator circuit. This is especially noticeable when only one receiver is being used, the output of which is connected to the eliminator circuit. The use of more than one receiver in combination with a key-click eliminator circuit of the type shown in Fig. 1 still further tends to make the applied signal voltage to the eliminator circuit more nearly approximate a D.C. voltage.

In determining the value of the condenser 35, care must be exercised to see that the value thereof is not too large. If it should be too large, it will be apt to discharge through the electron discharge device I! at the termination of a signal, and maintain conductivity through the tube so long as the energy stored up in the condenser flows therethrough. Such discharge through the tube i! will, as stated, occur after the cessation of the real signal, and in view of the exponential character of a condenser discharge, the discharge of the condenser 35 through the tube 11 will have the efiect of continuing the signal potential across the resistance 19, causing such signal to taper ofi in accordance with the discharge of the condenser.

Referring to Fig. 3 of the drawings, I have disclosed a practical application of my invention to commercial apparatus of the type known as a diversity receiver. Such apparatus has for its purpose to receive messages by radio substantially uninfiuenced by fading, and this is realized by receiving the signals si iultaneousiy upon a plurality of receivers 31, each having its separate antenna and each antenna located at a distance from the other. The output from each receiver 31 is coupled to a mixer circuit 33 including as many branches as there are receiving sets, each receiver being assigned to a branch circuit. Each branch comprises an electron discharge device 45 having an anode ti, cathode 4s, and control electrode 5i, and such other electrodes as may satisfactorily be employed in the circuit, the particular electron discharge device disclosed having, in addition to the above-mentioned electrodes, a suppressor grid 53 and a screen grid electrode 55. The output of each receiver 31 is coupled to the input circuit is of its associated electron discharge device :35, which input circuit is connected between the control grid 5! and the cathode ed, and includes a source of positive biasing potential e? common to all these input circuits and of a value sufficient to cause each tube 45 to swing into the region of saturation in response to the application of signals thereto, whereby in the output circuit is of each tube 45, which output circuit is connected between the anode ll and the cathode t9 and includes a source of anode potential 55 common to all the tubes 25, the signals will appear of uniform amplitude in amplified form. Such signals are fed from the output circuit of each tube 45 to a similar tube 6! having a cathode t3, anode s5 and a control electrode til in addition to a suppressor grid 69 and a screen grid electrode ii. The input circuit 13 to each tube iii, which is coupled to the output circuit as of the preceding tube it: to receive the amplified signals therein, is connected between the control elec-- trode 6'! and its associated cathode lit, and includes a source '55 of negative bias common to all tubes 8i and of a value sufiicient to normally bias each of these tubes 6i to cut-off in the absence of signals in the input circuit thereto. The signals impressed upon the input circuit of each tube ti from the output circuit of the preceding tube 55 are normally of su'fiicient magnitude to unbias the tube ti and permit plate current to flow therethrough. All the anodes 65 of the tubes iii are connected in parallel, thus providing a common output circuit ill, and when considered from the viewpoint of the circuit disclosed in Fig. 1, the tubes iii, taken collectively, correspond to the discharge device i of that circuit. By reason of the fact that the antennas 353 are located different separated points, the signals in the antenna circuits will not fade simultaneously, thus resulting in signals of more nearly uniform value in the common output circuit.

The above-mentioned biasing potential sources 51 and 15, and the potential source 59 for the anodes of the tubes t5, which also constitutes the source of screen grid voltage for all the tubes of the mixer circuit, are derived from a rectifier filter circuit of the customary type comprising a fullwave rectifier l9 and filter 5i, and terminating in a potentiometer from which the desired potentials may be obtained therein.

The key-click eliminator feature of the circuit, in accordance with the description of Fig. 1, comprises an electron discharge device 53 in the common output circuit of the above-mentioned tubes BI, and including an anode 85, cathode 81, and control electrode 6%]. This discharge device, as in the circuit of Fig. 1, is preferably of the gasfilled type, and is connected in series with a source of anode potential for the tubes 5 l, and a resistor 9|, to constitute the common output circuit of these tubes. The source of anode potential for the tubes 61, constitutes a rectifier filter circuit 93 along the lines previously mentioned, terminating in a potentiometer 95 from which the proper potential may be taken. Across a portion of this potentiometer is connected a resistor $7 from which suitable negative biasing potential of a value suflicient to normally maintain the discharge device 83 in an inoperative state, may be derived, the active portion of such biasing resistor 9'! being connected between the cathode 87 of the gas-filled discharge device 83 and its control electrode 89. In series therewith is connected an unbiasing resistor 99 having for its purpose the same function as the resistor 29 of Fig. 1, this resistor being shunted by a suitable condenser it! corresponding to condenser 3i of Fig. 1 to introduce the proper time factor, which will enable the circuit to difierentiate between desired and undesired signals in accordance with the explanation offered in connection with the operation of the circuit of Fig. 1. A resistor Hi3 connected between the grid 89 of the gas-filled tube 83 and the cathodest? of the tubes 6! corresponds to the resistor 33 of Fig. l, and completes the shunting circuit around the gas-filled discharge device 83.

The condenser 35 of Fig. 1 is represented as a phantom condenser H35 in the circuit of Fig. 3, for in all likelihood the stray capacities between the rectifier filter circuit as and ground will perform the function of condenser 35 of Fig. 1.

As will be seen from the description of the circu t thus far, the key-click eliminator feature embodies all the elements of the circuit of Fig. 1 but in a more commercial form.

In accordance with the explanation of the circuit as given in connection with the description of Fig. 1, the signals appearing across the resistance Si in series with the gas-filled discharge device 83 will represent the desired signals to the exclusion of the undesired signals such as keyclicks and the like. These desired signals are employed to control the operation ofa keying relay llil, or recorder, or any other suitable signal interpreting device or apparatus which may be connected across this resistor 9 l In order to satisfactorily control a relay keyer, the input to the keyer should approach square top impulses of pure direct current. A broadly tuned audio-frequency circuit comprising an inductance its and condenser ill, serially connected across the relay keyer itl, will function to by-pass most of the audio-frequency current component from the signals, leaving impulses approaching uniform direct current type to operate the relay keyer.

The presence of this filter circuit, however, tends to draw out the signal, but this can be corrected by shunting an additional circuit in parallel with the filter circuit, this additional circuit including an inductance H3 in series with a resistance I l5. By making this resistance I I5 variable, the degree to which the signal impulses may be extended by the filter circuit can be controlled, and this may be desirable in high speed keying where the duration of signal is quite short to begin with, and becomes still shorter, due to the delayed operation of the gas-filled tube 83 in respending to a signal, such delayed action causing a fractional portion of the signal to be lost. Such loss may be more than compensated for if desired, to assure satisfactory response of the relay keyer, by adjusting the resistance I I5.

While I have disclosed my invention primarily from the viewpoint of providing an improved circuit for the purpose of eliminating key-click interference, it will be appreciated from the very character of the invention and its mode of operation that any impulse type of interference of the order of key-clicks, such as may be encountered in practically all industrial areas, may in like manner be eliminated from received messages. Thus, ignition noises and many forms of static can be successfully kept out from the reproducing end of the receiver. It is, of course, also apparent that the same type of circuit may be adapted to suppress noise on speech circuits between sentences, words, or syllables, provided that power limiter amplifiers are employed. I accordingly do not desire to be limited in my protection to the specific details described above except as may be necessitated by the prior art and the appended claims.

1. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device responsive to transmit signals, a second electron discharge device capable of transmitting signal energy received from the said first electron discharge device, and means operated by signal energy transmitted by said first discharge device for biasing said second discharge device responsive to transmit signal energy received from said first device only after a predetermined portion of each desired signal has elapsed, said predetermined portion being of a time duration substantially equal to the maximum duration of an undesired signal and in time registry therewith.

2. A receiving circuit for discriminating between desired and undesired signals of shorter duration normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device responsive to transmit signals, a second electron discharge device capable of transmitting signal energy received from the operation of said first electron discharge device, and means operated by signal energy transmitted by said first discharge device for biasing said second discharge device responsive to transmit signal energy received from said first device only after an initial portion of a desired signal has elapsed, said initial portion being of a time duration substantially equal to the maximum duration of an undesired signal and in time registry therewith.

3. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device responsive to transmit signals, a second electron discharge device, and means normally biasing said second electron discharge device to an inoperative condition but responsive to signal ope-ration of said first discharge device for rendering said second discharge device responsive to transmit signals received from said first device after a predetermined period following application of each signal to said second discharge device, said predetermined period being of a time duration substantially equal to the maximum duration of an undesired signal and in time registry therewith.

4. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device having an anode, cathode, and a control electrode, a signal input circuit connected to said control electrode, an output circuit connected to said anode and including a second electron discharge device responsive to transmit signals received from said first discharge device, and means operated by signal energy transmitted by said first discharge device ior precluding operation of said second electron discharge device until after a signal has been applied thereto for a predetermined period of time substantially equal to the maximum duration of an undesired signal.

5. A receiving circuit for discriminating between desired signals and undesiredsignals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising an'electron discharge device having an anode, cathode, and a control electrode, a signal input circuit connected to said control electrode, an output circuit connected to said anode and including a second electron discharge device having an anode,-cathode, and a control electrode,'said second mentioned cathode being connected to the anode side of said first discharge device, whereas the anode of said second discharge device is connected to the cathode side of said first discharge device, means for normally precluding operation of said second electron discharge device, and means for rendering said second discharge device operative after said first device has been responsive to a signal for a predetermined period of time substantially equal to the maximum duration of an undesired signal and in time registry therewith.

6. Key-click elimination circuit comprising an electron discharge device having an anode, cathode, and a control electrode, a signal input circuit connected to said control electrode, an output circuit connected to said anode and including a second electron discharge device having a control electrode, means for applying a negative potential to said control electrode of a value sufficient to bias said second discharge device to an inoperative state, a resistance in series with said biasing means, a circuit shunting said second electron discharge device and including said resistance, whereby upon signal operation of said first electron discharge device, current in said output circuit will flow through said resistance and develop a voltage in opposition to said biasing potential, and means comprising a condenser shunting said resistance for delaying the development of such opposition voltage.

7. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device having a signal input circuit and a signal output circuit, a second electron discharge device in said output circuit normally inactive during non-signal responsive intervals of said first discharge device, and means responsive to flow of current in said signal output circuit upon the application of a signal in said signal input circuit for activating said second electron discharge device to transmit a signal received from said first discharge device after a predetermined time period of continuation of said signal, said predetermined period being substantially equal to the maximum duration of an undesired signal.

8. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device having a signal input circuit and a signal output circuit, a second electron discharge device in said output circuit normally inactive during nonsignal responsive intervals of said first discharge device, and means normally biasing said second device to an inoperative condition but responsive to flow of current in said signal output circuit upon the application of signals in said signal input circuit for activating said second electron discharge device to transmit signals received from said first discharge device after a predetermined time period of signal current flow in said signal output circuit substantially equal to the maximum duration of an undesired. signal.

9. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device having a signal input circuit and a signal output circuit, a second electron discharge device in said output circuit normally inactive during non-signal responsive intervals of said first discharge device, and means responsive to flow of current in said signal output circuit upon the application of a signal in said signal input circuit for activating said second electron discharge device to transmit after a lapse of predetermined time period, said means comprising a resistor-condenser combination in a branch portion of said output circuit paralleling said second electron discharge device.

10. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device having a signal input circuit, and a signal output circuit including an impedance of substantially infinite value, means by-passing said impedance and responsive to operation of said discharge device for suddenly changing said substantially infinite impedance to an impedance of very low value after a predetermined time interval, and a signal interpreting device actuable in response to such change in said impedance.

11. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising an electron discharge device having a signal input circuit, and a signal output circuit including an impedance of substantially infinite value, means by-passing said impedance and responsive to operation of said discharge device for suddenly changing said substantially infinite impedance to an impedance of very low value after a predetermined time interval, and means responsive to such change in said impedance for actuating a signal interpreting device.

12. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration otherwise normally receivable at the same frequency adjustment of the circuit, comprising a plurality of electron discharge devices each having its signal input circuit, and a common signal output circuit including an impedance of substantially infinite value, means by passing said impedance and responsive to signal operation of said discharge devices for suddenly changing said substantially infinite imedance to an impedance of very low value after a predetermined time interval of continuous signal operation, and a signal interpreting device actuable in response to such change in said impedance.

13. A receiving circuit for discriminating between desired signals and undesired signals of shorter duration occurring in synchronism with said desired signals, comprising an electron discharge device responsive to transmit both of said signals, a second electron discharge device responsive to transmit desired signals resulting from operation of said first electron discharge device, and means utilizing energy transmitted by said first discharge device for rendering said second discharge device non-responsive to said first device for the portion of each desired signal substantially coinciding with the time period of said undesired signal,

14. A system for receiving recurrent signals having an initial undesirable component comprising a first electron discharge device responsive to transmit said signals, a second electron discharge device responsive to transmit signals resulting from operation of said first electron discharge device, and a circuit coupling said devices for rendering said second discharge devices automatically non-responsive to said first device for an initial period of each desired signal, said period being of a time duration substantially equal to the maximum duration of said undesired component, said circuit coupling said devices for signal transmission for the remainder of said signal.

GILBERT W. CATTELL. 

